1. Field of the Invention
The present invention relates to a spread spectrum communication system for performing direct sequence code division multiple access to data having different symbol rates by a direct sequence spectrum spreading method. Especially, the present invention relates to a spread spectrum communication system by which it becomes possible to improve auto-correlation properties of code sequences, simplify initial synchronous capture characteristics and effectively allocate limited code resources.
2. Description of the Related Art
In recent years, according to increased demands for mobile communications, effective use of frequencies in mobile communications has become a major subject in developments. Then, Direct Sequence Code Division Multiple Access system employing a spread spectrum communication technique has been extensively investigated as a system for realizing efficient frequency allocation for practical use.
FIG. 9 is a principle diagram of direct sequence code division multiple access (CDMA) system employing a spread spectrum modulation technique. In FIG. 9, a specific code C.sub.i is multiplied to a digital signal S.sub.i of each channel on a sender SEND. If the frequency f and time t are respectively expressed in axises of an ordinate and an abscissa in FIG. 10, the frequencies are entirely spreaded through a band width. Because of that, all channel signal components may coexist, as shown in FIG. 10.
If the code C.sub.i used at the sender SEND is also multiplied to a receipt signal at a receiver REC, it is possible to extract only the original signal S.sub.i.
A direct sequence code division multiple access (CDMA) system using the spread spectrum modulation technique may be applied to a mobile cellular radio communication system, in which a plurality of base stations are arranged per cell, and signals to be sent to a plurality of users may be orthogonaized by the use of orthogonal codes, so that co-channel interference can be reduced in communications between base stations and mobile stations.
For example, a first technique for making sound signals orthogonaized by utilizing Walsh sequences disclosed in the Japanese unexamined patent application No. HEI6-501349 tilted "System and Method for Generating Signal Waveforms in a CDMA Cellar Telephone System" and a second technique for making signals orthogonaized by the use of orthogonal Gold codes described in the technical report No. RCS 95-79 in The Institute of Electronics, Information and Communication Engineers, titled "Coherent Multicode DS-CDMA Mobile Radio Access For Next Generation System" have been proposed.
On the other hand, demands for multimedia communications have been increased where various kinds of data, such as video image data, text data, and computer programs as well as sound data are transmitted and received is required. The multimedia communications have features such that required data transmission rate or communication quality is different, as depends on a type of communication data.
Therefore, to fulfill such the demands for multimedia communications it is required to have a system for achieving different data transmission rates. The orthogonal coding methods described in the first and second techniques are effective, when data have a same transmission rate. However, it is impossible to transmit data having different transmission rates, as keeping the data orthognized.
Therefore, on communications between the base stations and the mobile stations, interference has been generated between data having different data transmission rates. It causes a deterioration of communication quality.
To overcome the shortage of the above-described techniques, a third method has been proposed, in which codes to orthogonize signals having different spreading factors may be obtained by generating spreading codes with the tree structured generation method, which is described later, and the third method is disclosed in the technical report No. RC-96-103 of the Institute of Electronics information and communication, titled "Orthogonal Multi-Rate Forward Link Using Tree-Structured Generation Method of Orthogonal Spreading Codes for Coherent DS-CDMA.
In the third technique, the spreading codes are generated with the tree-structured generation method by the use of orthogonal Gold code sequences. However, the codes are structured by repeating short period codes, for example, 16 chip codes. Therefore, there is a problem that abnormal correlation values are periodically generated as shown in FIG. 11. That causes bad auto-correlation properties and deterioration of synchronous acquisition properties.